                    [:kg-per-m3 :lb-per-gal] #(/ % (factors [:lb-per-gal :kg-per-m3]))
                    [:m3-per-min :bbl-per-min] #(* % (factors [:m3 :bbl]))
                    [:bbl-per-min :m3-per-min] #(/ % (factors [:lb-per-gal :kg-per-m3]))
+                   [:lb-per-cu-ft :kg-per-m3] #(* % (factors [:lb-per-cu-ft :kg-per-m3]))
+                   [:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                    [:C :F] #(+ (* % 1.8) 32)}]
     (converter [from to])))
 

     :psi (tuc/draw-normal 7569.89 663.65)
     :kPa (tuc/draw-normal 64.1635 7.0987)
     :MPa (/ (typical-median-pressure :kPa) 1000))))

  (rand-nth physical-quantities))

(defn rand-unit [physical-quantity]
  (let [quantity-units-map {:density [:lb-per-cu-ft :kg-per-m3]
                            :length [:ft :m]
                            :mass [:lb :kg]
                            :pressure [:psi :kPa :MPa]
                            :proppant-concentration [:lb-per-gal :lb/gal :kg-per-m3 :kg/m3]
                            :slurry-rate [:bbl-per-min :bpm :m3-per-min :m3/min]
                            :volume [:bbl :m3]}]


    (fn [] (rand-nth (get quantity-units-map physical-quantity)))))

(def rand-density-unit (rand-unit :density))
(def rand-length-unit (rand-unit :length))
(def rand-mass-unit (rand-unit :mass))
(def rand-pressure-unit (rand-unit :pressure))
(def rand-proppant-concentration-unit (rand-unit :proppant-concentration))
(def rand-slurry-rate-unit (rand-unit :slurry-rate))
(def rand-volume-unit (rand-unit :volume))

(defn convert-units-f [from to]
  (let [factors {[:m :ft] 3.28084
                 [:kg :lb] 2.20462262185
                 [:m3 :bbl] 6.28981077
                 [:psi :kPa] 6.894757293168361
                 [:M :k] 1000
                 [:lb-per-gal :kg-per-m3] 119.826
                 [:lb-per-cu-ft :kg-per-m3] 16.0185}
        converter {[:ft :m] #(/ % (factors [:m :ft]))
                   [:m :ft] #(* % (factors [:m :ft]))
                   [:lb :kg] #(/ % (factors [:kg :lb]))
